Chris

Buy some reputable books on the subject, and have a read!

I don't mean that facetiously, but look through the 'Workshop Practice' series of books, everything you need is there, start with nos 3, 30, 34, 42 and 45.

Plastic is a wide description, – not all plastics are anything like the same in machining terms. There are 'brittle' plastics like polystyrene, and 'waxy' plastics like polythene, and 'rubbery' ones like polyurethane, but the nicest to machine include acetal, nylon, and rigid PVC. One problem is the nature of the turnings (which can be really tough) and the low melting points of plastics creating problems when parts rub together in machining, or when your tooling is the wrong shape or blunt.

Pure aluminium is a bit soft – the turning waste (or swarf) tends to push together into lumps, and it sticks to the tool, making a good finish difficult. But there are lots of light-alloys which turn nicely and some which polish nicely too. Just follow the advice – get hold of some reference books, which are often around second-hand or even seriously old (and rather dog-eared) so not very costly – and more recent ones for plastics, of course.

And practice.

Cheers, Tim